Study On The Dynamic Behavior Of Vapor Plume And Spatter During High-power Fiber Laser Welding

In high power fiber laser welding,evaporation of the irradiated material is very intense,the high temperature,high speed,and transient metallic vapor plume is difficult to flow out from the keyhole.And serious spatter defects occur with the intensive interaction between a high-power density laser beam and the material used in high power laser welding.Quantitative understanding the evolutions and process controlling mechanisms of metallic vapor plume and spatter dynamics in the dynamic keyhole could be served as a scientific basis for optimizing the quality of the heavy thick single pass welding joints.However,current theoretical modelling methods usually adopt experimental observation methods.Therefore,they cannot be used to accurately simulate the vapor plume behaviors.To solve the problems,this topic mainly carried out the following research:In this paper,an experimental platform for high-power fiber laser welding of stainless steel is constructed.The bead on plate experiments are conducted.The keyhole size,the flow of the molten pool and the appearance of the vapor plume and the evolution of the jet direction with time are studied.The measurement method of the vapor plume temperature and velocity at the keyhole is studied.Carry out "sandwich" welding experiments to observe the behavior of keyhole,molten pool and vapor plume during laser welding,and to qualitatively study the evolution of vapor plume dynamics in the keyholes.In addition,establish a three-dimensional mathematical model that can be used for high-power fiber laser welding,physically simulate the evolution of keyholes and molten pools over time in the welding process,reproduce the dynamic behavior of vapor plume and spatter during welding,and quantitatively reveal the evolution and regulation mechanism of vapor plume and spatter.In this paper,through a three-dimensional comprehensive model,it was directly observed that the intense localized boiling on the mesoscale keyhole wall induced many small spatters(LBi S),sized in the hundreds of microns,during 10 k W high power laser welding of 304 stainless steel.Experimental studies of parameters were also conducted to validate the model and characterize the spatter behaviors outside the keyhole wall during welding.The theoretical predictions of LBi S behaviors agreed well with the experimental observations.This finding shows that LBi S are very different from the commonly observed spatter defects produced by the intense friction effect of a vapor plume toward the keyhole,and this finding demonstrates the variety of the spatters.Furthermore,an energy conservation theory was proposed to characterize LBi S,and a novel dimensionless spatter number was formulated to calculate their formation threshold.The spatter number relates to the Reynold and Weber numbers of the molten flow of the weld pool and shows the relative importance of an inertia flow driven by the recoil pressure and the resistance from viscous dissipation,surface tension and gravity.The dimensionless spatter number was less than 1 when LBi S was formed.The proposed theory was validated against the numerical predictions,and good agreements were obtained.